This invention relates to a process for removing one or more toxic oxides forming undesirable contaminants in recovered waste dust and more particularly, to the removal of lead oxide and, when present, cadmium oxide and, when desired, zinc oxide by vaporization. The vaporization is essentially carried out in the oxidizing atmosphere at least at the vaporization temperature of lead oxide and, additional vaporization may be carried out in a reducing atmosphere at least at the vaporization temperature of zinc.
Melting or refining processes generate substantial quantities of dust which must be collected to prevent environmental pollution. Steel-making furnaces for example, liberate airborne particles which are typically collected by directing gases containing the particles to a filter situated in a bag house. Such steel-making facilities include an electric arc furnace, basic oxygen furnace, and an open hearth. While not so limited, the present invention is particularly useful to treat waste dust recovered from an electric arc steel-making facility in which it was a conventional practice to collect, filter, and dispose of all airborne particles as, for example, in a land fill operation. Such disposal is environmentally unacceptable because the waste dust contains leachable metal oxides of lead, cadmium, and chromium. It is an unnecessary economic burden on the steel-making process to dispose of the waste dust as a hazardous waste.
It is known in the art to pelletize steel-making dust and then heat the pellets in a reducing atmosphere to recover zinc by vaporization as disclosed in U.S. Pat. Nos. 3,756,804; 3,770,416; 4,266,966; 4,283,223; 4,396,423; 4,404,027; 4,434,001; and 4,488,905. The reduction process, however, cannot be utilized to remove lead and cadmium oxides which, if allowed to remain in the recovered mass from the reduction process, render the mass a hazardous material and importantly, prohibit recycling of the mass as a part of a furnace charge.